The present invention relates to a cutter such as a boring cutter, a holder for holding the cutter, and a tool assembly that includes the cutter and the tool holder.
Japanese Patent No. 2558054 discloses an annular cutter 106 having a shank 109 fitted in a supporting hole 101a of a supporting cylinder 101. FIGS. 9-13(d) represent the cutter of the Japanese patent. A sleeve 104 is rotatably supported at the tip of the supporting cylinder 101. As shown in FIGS. 11 and 12, a plurality of receiving holes 107a are defined in the supporting cylinder 101 so that they can be associated with a plurality of recesses 104a formed on the sleeve 104, respectively. Each receiving hole 107a contains a locking ball 107.
The supporting hole 101a contains a bearing piece 102. The bearing piece 102 is urged by a spring 110 toward the opening of the supporting cylinder 101. As shown in FIGS. 9 and 11, in the state where the shank 109 is not fitted in the supporting hole 101a, the balls 107 enter the respective recesses 104a of the sleeve 104 and are retained in this state by the bearing piece 102. The state of FIG. 9 is refered to as the unlocked state. The sleeve 104 is urged by a spring (not shown) in one rotational direction and is maintained in the state shown in FIGS. 9 and 11 when the balls 107 engage with the recesses 104a. 
As shown in FIGS. 13(a) and 13(b), the shank 109 has, on the outer circumferential face, a plurality of planar guiding faces 109a associated with the balls 107 respectively. Formed in each guiding face 109a is an engaging dent 109b that is engageable with the associated ball 107.
When the cutter 106 is fitted in the supporting cylinder 101, the shank 109 is inserted in the supporting hole 101a of the supporting cylinder 101, as shown in FIGS. 9 and 11, and the shank 109 pushes the bearing piece 102. Further, the engaging dents 109b are located at positions where they oppose the balls 107, respectively. Then, the spring turns the sleeve 104 with respect to the supporting cylinder 101 to push the balls 107 out from the recesses 104a. Thus, as shown in FIGS. 10 and 12, the balls 107 are engaged with the engaging dents 109b respectively to lock the cutter 106 against the supporting cylinder 101.
The cutter 106 is unlocked by turning the sleeve 104 against the urging force of the spring from the state shown in FIG. 12 to the state shown in FIG. 11.
In the unlocked state of FIGS. 9 and 11, the sleeve 104 is maintained in position with respect to the supporting cylinder 101 by the balls 107, which are held in position by the bearing piece 102. The balls 107 can move slightly within the recesses 104a of the sleeve 104 and within the receiving holes 107a of the supporting cylinder 101. Thus, in the unlocked state, the sleeve 104 is not firmly locked with respect to the supporting cylinder 101, making the position of the sleeve 104 unstable. This makes it difficult to stabilize the balls 107 in the unlocked state.
The bearing piece 102 is located in the supporting hole 101a of the supporting cylinder 101. This makes the internal structure of the supporting cylinder 101 complicated.
The outside diameter of the shank 109 is substantially equal to the inside diameter of the receiving hole 101a of the supporting cylinder 101. This is so that the shank 109 is held firmly in the supporting cylinder 101 with no slack. However, this makes it difficult to insert the shank 109 into the receiving hole 101a. 
As shown in FIG. 11, in the unlocked state, the balls 107 slightly protrude radially inward from the inner circumferential surface of the receiving hole 101a. Thus, when the shank 109 is inserted in the supporting cylinder 101, the end face of the shank 109 engages against the balls 107 unless the guiding faces 109a are aligned with the balls 107. This prevents smooth insertion of the shank 109 into the receiving hole 101a and also damages the balls 107. In order to solve this problem, it is essential to prevent the balls 107 from protruding radially inward from the inner circumferential surface of the receiving hole 101a, in the unlocked state. However, this limits the degree of freedom in designing the supporting cylinder 101 and 109.
Japanese Patent No. 2558054 discloses another cutter 106 as shown in FIGS. 13(c) and 13(d). In this cutter 106, the shank 109 has a large-diameter portion 109c and a small-diameter portion 109d. A step 109e is formed between the large-diameter portion 109c and the small-diameter portion 109d. The diameter of the large-diameter portion 109c is substantially equal to the diameter of the receiving hole 101a of the supporting cylinder 101. The small-diameter portion 109d facilitates the operation of inserting the shank 109 into the supporting hole 101a. Each engaging dent 109b is formed on both the large-diameter portion 109c and the small-diameter portion 109d. 
In the cutter 106 shown in FIG. 13(b), the step 109e engages the locking balls 107 unless the engaging dents 109b and the balls 107 are aligned. Therefore, to avoid such engagement, it is essential to prevent the balls 107 from protruding radially inward from the inner circumferential surface of the receiving hole 101a, in the unlocked state. This limits the degree of freedom in designing the supporting cylinder 101 and the shank 109, as in the case of the cutter of FIG. 13(a).
It is an objective of the present invention to position securely the operating body with respect to the supporting cylinder.
It is another objective of the present invention to facilitate smooth insertion of the cutter into the supporting cylinder and also to increase the degree of freedom in designing the supporting cylinder and the cutter.
To achieve the above objective, the present invention provides a tool holder for holding a shank of a cutter tool. The tool holder comprises a supporting cylinder. A supporting hole that receives the shank is formed in the supporting cylinder. A locking body moves between a locking position and an unlocking position. A locking member is supported by the supporting cylinder. When the locking body is located in the locking position, the locking member is moved radially inward with respect to the supporting cylinder by the body to engage with the shank. When the locking body is located in the unlocking position, the locking member is allowed to move in the radially outward direction of the supporting cylinder to release the shank. An engaging member is located between the supporting cylinder and the locking body. The engaging member moves between an engaging position, at which the engaging member locks the locking body with respect to the supporting cylinder, and a disengaging position, at which the engaging member releases the locking body from the supporting cylinder. When the engaging member is in the engaging position, the locking operation body is prevented from moving relative to the supporting cylinder and is held in the unlocking position, and when the engaging member is in the disengaging position, the locking operation body is allowed to move relative to the supporting cylinder.
To achieve the above objective, the present invention also provides a cutter tool that fits in a tool holder. The tool holder has a supporting hole that receives a shank of the cutter and a locking member for engaging the shank such that the locking member holds the shank in the supporting hole. The cutter tool comprises a large diameter section for contacting an inner wall that defines the supporting hole. A small diameter section, the diameter of which is smaller than that of the large diameter section. The small diameter section is adjacent to the large diameter section and is located at the distal end of the shank. A recess is located entirely in the small diameter section, and the locking member enters the recess.
To achieve the above objective, the present invention also provides a tool assembly. The tool assembly omprises a cutter tool having a shank. The shank has a large diameter section and a small diameter section. The small diameter section is adjacent to the large diameter section and is located at the distal end of the shank. A tool holder is attaching the shank. The tool holder comprises a supporting cylinder. The supporting cylinder has a supporting hole being inserted in the shank of the cutter. An inner wall that defines the supporting hole has a first inner wall corresponding to the small diameter section of the shank and a second inner wall contacting with the large diameter section of the shank. The diameter of the first inner wall is smaller than the diameter of the second inner wall. A locking body is supported to move between a locking position and an unlocking position. A locking member is supported by the supporting cylinder to move in the radial direction. When the locking operation body is located in the locking position, the locking member moves to an inner side of radial direction by the locking body to engage with the shank. When the locking body is located in the unlocking position, the locking member is allowed to move to an outer side of radial direction to release from the shank.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.